The invention relates to a device for controlling at least one variable influencing the drive torque of a motor vehicle using wheel slip and a control variable which is the difference between wheel acceleration and vehicle longitudinal acceleration.
A device having these features is known from EPO 229,249Al. According to this patent, for a vehicle with all-wheel drive, the slip of driven wheel is calculated from the difference between wheel acceleration and vehicle longitudinal acceleration, since non-driven wheels are not present for determining the vehicle speed and hence for the slip of the driven wheels. Control is performed by comparing the slip value calculated with a predetermined threshold value.
Known propulsion control systems generally use the control variables slip or acceleration of driven wheels, control being initiated after a slip threshold is exceeded. It is also known to change the slip threshold, for example when driving in curves or when driving with snow chains, in order to improve the traction or cornering force respectively.
Control is generally activated quickly when the road surface becomes suddenly worse, for example when it changes from dry to wet or icy sections of road. Conversely, this has not been the case until now when conditions suddenly improve, since an increase of the drive torque is made only relatively slowly in order not to enter the next control cycle with too great an excess torque.
It is the object of the invention to create a control device for vehicles with single-axle drive which adapts automatically to changing adhesion coefficients, reacts quickly to rising adhesion coefficients and permits an optimum vehicle acceleration.
This object is achieved as follows.
According to the present invention, the difference between wheel acceleration and vehicle longitudinal acceleration is used as a control variable and compared with a control threshold value. This means that where the vehicle longitudinal acceleration is great, a greater wheel acceleration is necessary in order to activate a control process. As a result, an acceleration process of the vehicle is not essentially hindered by a control process. The slip of the driven wheels serves as a control variable for the effectiveness of the control. The changing of the slip threshold, depending on the vehicle acceleration in conjunction with time windows, and the drive torque increase, depending on the ratio of speed and the current vehicle longitudinal acceleration, allows the control to be rapidly adapted to a so-called micro-jump in the direction of an increasing adhesion coefficient.
By automatically tracking the threshold values for slip and acceleration differences as well as the duration of the time windows, control of the drive torque, which is adaptive and rapidly takes into account the current road conditions, is obtained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.